Polar Opposites

Can I kick my sister's ass? She just drives me insane. I told my father that I'm so mad at her and he says, "Why?" He's so right. She didn't do anything to me. I just seriously wonder about her priorities.
She has a daughter (my niece) and she's always unloading her when she can whether it's on me, my grandmother, or my mother.
I was proud of her a few weeks ago when she dumped her boyfriend because he cheated on her. She said she couldn't accept his behavior and what kind of example would she give to her daughter? She seemed so wise that one night. I told her that her words were those of a person that I would respect and look up to, not as a sister, but as an outsider.
Last night, she hooked up with him again and dropped my niece off at my mom's.
sigh.
Another thing, she's always broke and asking me to lend her money, but then she goes out to clubs and drinks.